Photovoltaic cell

ABSTRACT

A photovoltaic cell has two electrodes and a hole injection layer. A liquid crystal material and a plurality of particles can be disposed between the electrodes.

CROSS REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119(e), this application claims priority to U.S.Provisional Application Ser. No. 60/664,298, filed Mar. 22, 2005, and toU.S. Provisional Application Ser. No. 60/664,336, filed Mar. 23, 2005,the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The invention relates to photoactive layers that contain liquid crystalmaterials, as well as related photovoltaic cells, systems and methods.

BACKGROUND

Photovoltaic cells, sometimes called solar cells, can convert light,such as sunlight, into electrical energy.

SUMMARY

The invention relates to photoactive layers that contain liquid crystalmaterials, as well as related photovoltaic cells, systems and methods.

In one aspect, the invention relates to a photovoltaic cell including afirst electrode and a second electrode. A hole injection layer, a liquidcrystal (LC) material, and a plurality of particles in electricalcontact with the liquid crystal material are disposed between the firstand second electrodes.

In some embodiments, the particles are inorganic particles. Theparticles may be elongated.

In some embodiments, a plurality of surface groups is bound to at leastsome of the particles. The particle surface groups may be electroactive.The particle surface groups can increase an ability of the LC materialto wet the particles.

In some embodiments, at least some of the particle surface groupsinclude a plurality of aromatic rings. At least some of the particlesurface groups can include a plurality of 5-membered rings. At leastsome of the 5-membered rings can include at least one sulfur atom. Atleast some of the particle surface groups can include at least onesulfoxide group. In some embodiments, one of the 5-membered rings of atleast some of the particle surface groups is a terminal 5-membered ringand only the terminal 5-membered ring includes a sulfoxide group.

In some embodiments, at least some of the particle surface groupsinclude at least one aromatic ring having an aliphatic chain of at least3 carbon atoms.

In some embodiments, the LC material is an electroactive LC material.The LC material can be a nematic LC material. The nematic LC materialcan be a discotic nematic LC material.

In some embodiments, the LC material includes a plurality of LC unitsand at least some of the LC units are in different layers of the LCmaterial. The positions of the LC units in different layers may besubstantially random.

In some embodiments, the liquid crystal material defines a director(e.g., a layer director) and the particles define a major axis longerthan a minor axis of the particles. The director and the major axes ofsubstantially all the particles may be substantially aligned.

In some embodiments, the liquid crystal material defines a director(e.g., a layer director) and the hole injection layer comprises aplurality of polymer rods in contact with the liquid crystal material.The director and substantially all of the polymer rods may besubstantially aligned.

In some embodiments, the liquid crystal material includes a plurality ofdiscotic LC units and each LC unit has an aromatic central group and atleast 4 electroactive arms. The aromatic central group can include atleast 2 fused aromatic 6-membered rings and at least 4 heterocyclicrings fused to the aromatic 6-membered rings. The aromatic central groupcan include at least 3 fused aromatic 6-membered rings. At least some ofthe heterocyclic rings may be 5-membered rings having a sulfur atom. Theelectroactive arms can include a plurality of 5-membered rings. At leastsome of the 5-membered rings can include a sulfur atom. At least one ofthe 5-membered rings of each electroactive arm can include an aliphaticchain having at least 3 carbon atoms.

In some embodiments, the LC material includes or is formed entirely ofLC units that are free of metal atoms.

In some embodiments, a photoactive binder is dispersed among the liquidcrystal material. The photoactive binder can include a polymer that hasa LUMO similar to a LUMO of the LC material. The photoactive binder canbe a polymer that has a HOMO similar to a HOMO of the LC material.

In some embodiments, the first electrode is a mesh electrode.

In some embodiments, the particles are formed of a material selectedfrom the group consisting of ZnO, WO3, and TiO2. The particles may beelongated and with a first end of at least some of the particles fixedwith respect to one of the electrodes and a second end of the at leastsome of the particles free of the electrode.

Another aspect of the invention relates to a method for manufacturing aphotovoltaic cell. In some embodiments, the method includes disposing ahole injection material, a plurality of particles, at least some ofwhich have a particle surface group associated therewith, and aplurality of liquid crystal (LC) units between first and secondelectrodes.

In some embodiments, the method includes forming a discotic nematic LCmaterial including the LC units. Forming the discotic nematic LCmaterial can include forming a layer of the hole injection material, thelayer having a surface, associating a surfactant with a surface of thelayer of the hole injection material, and contacting the surfactant withthe LC units.

Another aspect of the invention relates to discotic nematic liquidcrystal (LC) material. In some embodiments, the LC material includes aplurality of LC units, with each LC unit having a central moietycomprising at least one aromatic ring and a plurality of electroactivearms extending outward from the central moiety, each arm comprising aplurality of cyclic groups at least some of which comprise at least onesulfur atom.

Other features and advantages will be apparent from the description,drawings, and claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of an embodiment of a photovoltaiccell.

FIG. 2 is a partial cross-sectional view of the photovoltaic cell ofFIG. 1.

FIGS. 3 a-3 c illustrate embodiments of units of conducting polymers ofphotoactive material.

FIG. 4 is a schematic illustration of liquid crystal material of thephotovoltaic cell of FIG. 1.

FIGS. 5 a-5 e illustrate embodiments of units of liquid crystalmaterials.

FIGS. 6 a-6 f illustrate embodiments of central groups of units ofliquid crystal materials.

FIG. 7 illustrates an embodiment of a method for preparing a bindinggroup.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1 shows a photovoltaic cell 100 that includes an electrode 101 anda counterelectrode 103 that are electrically connected to an externalload 105. Electrode 101 includes a substrate 107 and an electricallyconductive layer 111. Photovoltaic cell 100 also includes a photoactivelayer 113 and a hole injection layer 115 disposed between electrode 101and counterelectrode 103.

Photoactive layer 113 includes a semiconductor material and anelectroactive liquid crystal (LC) material 200. The semiconductormaterial is formed of particles 119 of the semiconductor material.Electroactive LC material 200 is formed of a plurality of LC units 201.Photoactive layer 113 further includes a plurality of particle surfacegroups 121, at least some of which are associated with (e.g., physicallyand/or chemically sorbed to and/or bound to) particles 119 of thesemiconductor material. Photoactive layer also includes a photoactivebinding material 117 disposed between particles 119 and LC units 201.

During operation, in response to illumination by radiation (e.g., lightin the solar spectrum), photovoltaic cell 100 produces a flow ofelectrons across load 105. The illuminating radiation is absorbed by andproduces excitons within the electroactive LC material 200, photoactivebinding material 117, and/or particle surface groups 121. The excitonsmigrate through the LC material 200, binding material 117, and/orparticle surface groups 121 to particles 119. At the particles, electrontransfer occurs when an electron from an exciton is transferred to aparticle leaving behind a hole within the surrounding photoactive layer.Electrons pass into the conduction band of the particles, flow throughthe particles to counter electrode 103, and then to external load 105.After flowing through external load 105, the electrons flow to electrode111, to layer 115, and then return to photoactive layer 113. Holesgenerated by the electron transfer at the particles migrate, e.g., byelectron exchange, through photoactive layer 113 to hole injection layer115, where the holes can recombine with electrons returning tophotoactive layer 113. Electron transfer and electron-hole recombinationoccurs continuously during illumination so that photovoltaic cell 100can provide continuous electrical energy to external load 105.

Referring also to FIG. 2, particles 119 define a major axis a₁ and aminor axis a₂ (i.e., the particles are elongated). In some embodiments,the average length l₁ of the particles along major axis a₁ is at leastabout 100 nanometers (e.g., at least about 200 nanometers, at leastabout 250 nanometers, at least about 350 nanometers). In certainembodiments, the average length l₁ is at most about 1000 nanometers(e.g., at most about 750 nanometers, at most about 500 nanometers, atmost about 400 nanometers, at most about 300 nanometers). In someembodiments, the average width w₁ of the particles along minor axis a₂is at least about 2 nanometers (e.g., at least about 3 nanometers, atleast about 5 nanometers, at least about 7 nanometers). In certainembodiments, the average width w₁ is at most about 50 nanometers (e.g.,at most about 35 nanometers, at most about 25 nanometers, at most about15 nanometers, at most about 10 nanometers). For example, in certainembodiments, the particles can have an average length l₁ of from about100 nanometers to about 600 nanometers (e.g., from about 200 nanometersto about 500 nanometers) and an average width w₁ of from about 2.5 toabout 50 nanometers (e.g., from about 5 nanometers to about 20nanometers).

Particles 119 have a free end 143 and a fixed end 141, which is fixedwith respect to a surface 131 of counterelectrode 103. Major axes a₁ ofparticles 119 are aligned at least in part along a common axis, which isoriented normal to inner surface 131. For example, as seen in FIG. 2,major axes a₁ of particles 119 are generally aligned with the z-axiswhile inner surface 131 occupies an x-y plane normal to the z axis.Without being bound by theory, it is believed that such orientationfacilitates electron transfer along particles 119 to counterelectrode103.

In some embodiments, the average distance d₁ (FIG. 1) between the majoraxes a₁ of nearest particles is at most about 150 nanometers (e.g., atmost about 100 nanometers, at most about 75 nanometers, at most about 50nanometers, at most about 35 nanometers, at most about 10 nanometers).Without wishing to be bound by theory, it is believed that averagedistances of about 50 nanometers or less (e.g., of about 20 nanometersor less) enhance the efficiency of exciton transfer within photoactivelayer 113 by reducing the likelihood that an exciton will be quenchedbefore reaching the particles.

Examples of semiconductor materials from which particles 119 can beformed can have the formula M_(x)O_(y), where M may be, for example,titanium, zirconium, zinc, tungsten, niobium, lanthanum, tantalum,terbium, or tin and x is greater than zero (e.g., an integer greaterthan zero) and y is greater than zero (e.g., an integer greater thanzero). Additional examples of semiconductor materials from which theelongated particles can be formed include sulfides, selenides,tellurides, and oxides of titanium, zirconium, niobium, lanthanum,tantalum, terbium, tin, zinc or combinations thereof. For example, TiO₂,SrTiO₃, CaTiO₃, ZrO₂, WO₃, La₂O₃, Nb₂O₅, SnO₂, ZnO, sodium titanate,cadmium selenide (CdSe), cadmium sulphides, and potassium niobate may besuitable materials.

Semiconductor materials from which particles 119 can be formed canexhibit electron mobilities of at least about 1×10⁻³ cm²V⁻¹s⁻¹ (e.g., atleast about 10² cm²V²⁵s⁻¹).

While particles 119 are shown as elongated along a major axis, theparticles may have other shapes. For example, the particles may becurved, fibrous, branched, or spherical. In some embodiments, theparticles may be tubular or shell-like.

In general, particles 119 can be prepared as desired. Examples ofpreparation methods include templated synthesis, direct synthesis of anarray of oriented particles, and hydrothermal synthesis.

Typically, templated synthesis includes growing particles 119 from asurface that has a plurality of nucleation sites, which are generallyformed of the same material as particles 119. For example, nucleationsites to grow ZnO particles can be prepared as follows. ZnOnanoparticles (Alfa Aesar, Ward Hill, Mass.) are dispersed in a solvent(e.g., water). The nanoparticles can have an average diameter of fromabout 10 nm to about 100 nm (e.g., from about 25 nanometers to about 75nanometers). A dispersant (e.g., a polyacrylic-based dispersant) isadded to the nanoparticles to form a suspension. The suspension is mixedand centrifuged to remove agglomerated nanoparticles. The centrifugedsuspension is applied as a thin coating to a conductive surface (e.g.,the surface 131 of counterelectrode 103). The coating is dried (e.g., inair at room temperature) to produce a plurality of ZnO nucleation siteson the surface of the substrate.

In some embodiments, the conductive surface is formed of a metal (e.g.,copper, aluminum, titanium, indium, or gold). For example, theconductive surface can be the surface of a metal foil. In certainembodiments, the surface is a metal oxide, such as indium tin oxide(ITO), tin oxide, a fluorine-doped tin oxide, and zinc-oxide. The oxidesurface can be supported by, for example, a metal foil or a polymer.

To prepare particles 119, the nucleation sites are contacted with anaqueous solution of zinc (e.g., as Zn(NO₃)₂) and an amine (e.g.,hexamethyltetramine (HMT)). Typically, the zinc concentration is fromabout 0.005 to about 0.08 M (e.g., from about 0.01 to about 0.05 M).Typically, the HMT concentration is from about 0.0005 M to about 0.005 M(e.g., from about 0.001 M to about 0.002 M). The solution is allowed toreact with the nucleation sites for a period of time sufficient to groworiented ZnO particles. In general, the time is from about 24 hours toabout 96 hours (e.g., from about 48 hours to about 80 hours, such asabout 72 hours). In general, the temperature is from about 40° C. toabout 80° C. (e.g., from about 50° C. to about 70° C., such as about 60°C.). The resulting particles 119 can be washed (e.g., by rinsing with asolvent such as water) to remove unreacted zinc and amine. The particlesare oriented with respect to the surface (e.g., with respect to surface131 of counterelectrode 103 as seen in FIG. 1).

We turn next to particle surface group 119. In some embodiments, theparticle surface group can modify (e.g., increase) the ability of thephotoactive material to wet particles 119. Alternatively, or inaddition, the particle surface groups can enhance charge conductionbetween the LC material and photoactive binder and particles 119. Asseen in FIG. 2, particle surface group 121 is formed of a conductingpolymer 122, a light absorption modifier 123, a plurality of pendentsolubilizing groups 124, and a binding group 126.

In general, conducting polymer 122 includes a plurality of units. InFIG. 2, conducting polymer 122 is a polythiophene polymer having 5 units(e.g., thiophene rings). In some embodiments, conducting polymer 122includes fewer units (e.g., 2 units, 3 units or 4 units). In certainembodiments, conducting polymer 122 includes more than units (e.g., 6units, 7 units, 8 units, 9 units, 10 units, or more).

Additional examples of units from which the conducting polymers can beformed are shown in FIGS. 3 a-3 c. In FIG. 3 a, a unit 300 includes apair of thiophene rings that define a third ring between them. A pair ofpendent solubilizing groups extend from the third ring. In FIG. 3 b, aunit 302 includes a pair of phenyl groups that define a third ringbetween them. In FIG. 3 c, a unit 304 includes a plurality of pendentheterocyclic and phenyl groups.

Further examples of conducting polymers from which the particle surfacegroups can be formed include polyaniline (Pan), poly(N-vinycarbazole)(PVCZ), polyacetylene (PA), polypyrole (PPy), poly(2-vinylpyridine)(P2VP), and poly(p-phenylenevinylene) (PPV) and other polyphenylenes,poly-(3-hexylthiophene), polyphenylacetylene, polydiphenylacetylene,oligothiophenes, and combinations thereof. Derivatives of suchconducting polymers can also be used. For example, bridged thiopheneshaving two thiophene polymers linked together by a bridging group can beused. In some embodiments, the particle surface groups are LC units,such as units of a discotic nematic liquid crystal material as discussedbelow.

In general, conducting polymer 122 is able to transfer electrons alongat least a portion of its length (e.g., between LC material 200 andparticles 119). In some embodiments, the energy level of the lowestunoccupied molecular orbital (LUMO) of the particle surface group ishigher than the energy of the conduction band of particles 119. Withoutwishing to be bound by theory, it is believed that the higher LUMO ofthe particle surface group can enhance electron transfer from theparticle surface group to the particles. Hence, particle surface group121 can increase the performance of cell 100 and/or can facilitate(e.g., enhance) electron transfer from LC material 200 and/orphotoactive binder 117 to particles 119.

Light absorption modifier 123 is a sulfoxide group that shifts the rangeof light absorbed by the particle surface group 121 to longerwavelengths. This shift increases the amount of visible solar radiationabsorbed by the particle surface group 121. For example, particlesurface group 121 with the sulfoxide group has an absorption maximumbetween about 500 nanometers and about 600 nanometers, which range isabout 50 nanometers to about 100 nanometers higher than if the sulfoxidegroup were not present.

In FIG. 2, the light absorption modifier is located on the terminalthiophene ring 125. In certain embodiments, some or all of the otherunits (e.g., some or all of the other thiophene rings) of particlesurface group 121 may also or alternatively include a light absorptionmodifier. In some embodiments, no light absorption modifier is present.

Solubilizing groups 124 can enhance the solubility of particles surfacegroup 121 with respective the photoactive material of layer 113. Asshown in FIG. 2, solubilizing groups 124 are alkyl chains. At least some(e.g., all) units of particle surface group 121 include a respectivependent alkyl chain. In general, at least some (e.g., all) of thependent groups are alkyl chains at least 3 atoms long (e.g., at least 4atoms long, at least 5 atoms long, at least 6 atoms long, or more). Atleast some (e.g., all) of the atoms may be carbon atoms. Othersolubilizing groups can be used. Examples of other solubilizing groupsinclude branched alkyl groups, alkoxy groups, cyano groups, aminogroups, and/or hydroxy groups. As seen in FIGS. 3 a and 3 b, each unitof a particle surface group can have more than one (e.g., 2, 3, or more)solubilizing groups.

Binder group 126 associates particle surface group 121 with its particle119. Binder group 126 includes a phosphonic acid group having a highaffinity for the semiconductor material of particle 119. Without wishingto be bound by theory, it is believed that the phosphonic acid groupallows (e.g., does not substantially inhibit) efficient electrontransfer between conducting polymer 122 and particle 119.

Particles 119 can include a plurality of particle surface groups. Insome embodiments, the average number of particle surface groups perparticle is at least about 10 (e.g., at least about 1000, at least about5,000, at least about 10,000). In some embodiments, at least about 10%(e.g., at least about 25%, at least about 50%, at least about 75%) ofthe particle surface area is covered by surface groups. For example, insome embodiments, a surface area of the particles is between about 75and 80 m²/g and surface groups 121 are present at about 60 mg surfacegroup per gram particle, which corresponds to about 60% coverage of theparticle surface. In some embodiments, the particles surfaces arecompletely covered.

In general, particle surface group 121 can be prepared as desired.Typically, a polythiophene polymer is polymerized from halogentatedthiophene monomers that are substituted with a pendent solubilizinggroup. The binder group is added to the polythiophene polymer.Typically, the method for adding the binder group includes brominatingthe polythiophene polymer and converting the bromine to a phosphonate(e.g., a phosphonate ester) by a nucleophilic displacement reaction. Thephosphonate is transformed into the phosphonic acid to provide particlesurface group 121.

While binder group 126 includes a phosphonic acid group, other bindergroups can be prepared. Examples of other binder groups includephosphine oxide groups, amino groups, carboxy groups, and thiol groups.Referring to FIG. 7, preparation of a particle surface group having acarboxy binder group can include preparation of a Grignard reagent(e.g., 2-thiophene magnesium bromide) (as indicated by step 1),synthesis of 3,3′″-dihexyl-[2,2′;5′,2″;5″,2′″]quaterthiophene (asindicated by step 2), stannylation of3,3′″-dihexyl-[2,2′;5′,2″;5″,2′″]quaterthiophene (as indicated by step3), coupling of the stannylated3,3′″-dihexyl-[2,2′;5′,2″;5″,2′″]quarterthio phene with5-bromo-2-thiophene carboxylate (as indicated by step 4), and hydrolysisof the monoester to the corresponding carboxylic acid (as indicated bystep 5). The order of at least some of these steps can be changed, stepscan also be combined, and some steps can be eliminated (e.g.,preparation of the Grignard reagent).

A Grignard reagent can be prepared as desired. Typically, an amount ofmagnesium (e.g., magnesium turnings) are combined with a solvent (e.g.,ethyl ether) in a reaction flask. In an exemplary embodiment, about 1.07g magnesium (e.g., about 0.44 mmoles) is combined with about 15 mL ethylether. While the magnesium is stirred slowly under N₂, dropwise additionof a solution of 2-bromo-3-hexyl-thiophene, 9 g. (36.4 mmoles) in 50 mLethyl ether is performed. After addition of the bromide, the system isrefluxed for 2 hours. The 2-thiophene magnesium bromide Grignard thusformed is used below.

The Grignard reagent prepared above, added dropwise to a suspension of5.0 g (15.4 mmoles) of 5,5′-dibromo-2,2′-bithiophene and 0.084 g (0.154mmoles) of Ni(dppp)₂Cl₂ in 50 mL dry ethyl ether. Typically, theGrignard is added from a feeding funnel under an inert gas (e.g., N₂).The resulting mixture was heated to reflux for about 20 hours. Theresulting mixture is cooled to about room temperature (e.g., to about20° C.) and combined with about 200 mL of 5% aqueous NH₄Cl to which someice is added, to effect hydrolysis. The aqueous phase is extracted withabout 2×100 mL of CH₂Cl₂. The organic phases combined are back washedwith about 200 mL water and dried over MgSO₄. The solvent is evaporatedto give a raw product, 3,3′″-dihexyl-[2,2′;5′,2″;5″,2′″]quaterthiophene,which can be purified using, for example, a silica-gel column usinghexanes 100% as eluant to give 6.4 g of material, (yield=83.4%). ProtonNMR can be used to determine the presence of the raw product: ¹HNMR:7.25 ppm, (2H, d), 7.20 ppm, (2H, d), 7.10 ppm, (2H, d), 7.00 ppm, (2H,d), 2.83 ppm, (4H, t), 2.7 ppm, (4H, m), 1.41 ppm, (12H, m), 1.00 ppm,(6H, m).

The 3,3′″-dihexyl-[2,2′;5′,2″;5″,2′″]quaterthiophene can be stannylatedas desired. Typically, an amount of3,3′″-Dihexyl-[2,2′;5′,2″;5″,2′″]quaterthiophene (e.g., 2.990 g, (6mmoles) is dissolved in 50 mL of dry THF and cooled to about 0° C. About3.0 mL of lithium diisopropyl amide, (LDA) 2.0M inheptane/tetrahydrofuran/ethylbenzene are added dropwise (e.g., using asyringe). After this addition, the system is stirred for an additionalhour and then 1.6 mL of Bu₃SnCl (6.0 mmoles) is added. This mixture isstirred for about 45 minutes at about 0° C. and then for about 1 hour atabout room temperature (e.g., at about 20° C.). Solvent from theresulting mixture is evaporated (e.g., using a Rotovap. About 100 mL ofwater and about 100 mL of CH₂Cl₂ are added to the mixture from which thesolvent was evaporated. The resulting mixture is combined well (e.g., byshaking in a separatory funnel). Typically, the organic phase separatesand the aqueous phase is washed with about 100 mL CH₂Cl₂. The organicphases are combined and dried over Na₂SO4. The solvent was removed togive a stannylated product that was used without further purification.

The stannylated 3,3′″-dihexyl-[2,2′;5′,2″;5″,2′″]quarterthio phene canbe coupled with with 5-bromo-2-thiophene carboxylate as desired.Typically, 5-Bromo-2-thiophene carboxylate, 1.4 g, (6.33 mmoles) and5.156 g (6.5 mmoles) of the stannylated3,3′″-dihexyl-[2,2′;5′,2″;5″,2′″]quarterthiophene are combined andpurged with an inert gase (e.g., N₂). Cessium fluoride (CsF) about 3.1g, (20 mmoles) is added followed by about 40 mL of dryN-methylpyrrolidinone (NMP). The mixture is purged with additional N₂and Pd₂ dba₃ (about 0.406 g, 0.44 mmoles) and P(t-Bu)₃ (about 3.0 mL of10% hexanes solution) is added. The mixture is stirred at roomtemperature (e.g., about 20°) for a period of time (e.g., for about 18hours).

About 200 mL is added to the stirred mixture. The resulting mixture isextracted with 2× about 150 mL CH₂Cl₂. The organic phases are combinedand dried over MgSO4. The solvents are removed (e.g., using a Rotavap).The residue remaining after solvent removal can be chromatographed(e.g., on silica-gel using hexanes/CH₂Cl₂ mixture with variedcomposition from 100/0 to 70/30 v/v %). Typically, the product is elutedwith hexanes/CH₂Cl₂ 80/20 as a third fraction. The coupled product canbe recrystallized (e.g., from hexanes) to remove NMP. Typically, about0.546 g. of mono and the doubly coupled materials are obtained. The monoand the doubly coupled materials can be separated using, for example, asilica-gel column with hexanes/ethyl acetate 80/20 v/v % as eluant, toobtain about 0.252 g of the monoester and about 0.108 g of the diester.The methyl ester group appears about about 3.95 using ¹H NMRspectroscopy.

The monoester prepared above can be hydrolyzed to the correspondingcarboxylic acid as desired. In some embodiments, about 0.252 g, (0.39mmoles) is dispersed in about 15 mL of a 0.4M NaOH ethanolic solutionand heated to about 60° C. for a period of time (e.g., about 4 hours).The resulting mixture is combined with (e.g., poured into) about 100 mL0.2 M aqueous HCl. A precipitate is collected by filtration and driedfor a period of time (e.g., about 5 hours) at about 60° C. under reducedpressure. About 0.172 g of a pentatiophene carboxylic acid is typicallyobtained.

Proton NMR typically indicates that the methyl ester peak at 3.95 ppm isreduced or eliminated. FT-IR spectroscopy typically indicates thepresence of a carboxylic group at about 1710 cm⁻¹. ¹³C NMR typicallyindicates the presence of a carboxylic acid group (e.g., by a peak atabout 165 ppm). Ultraviolet visible spectroscopy typically indicates apeak at about 422 nanometers with an extinction coefficient of about33644 L cm per mol in CHCl₃.

Without wishing to be bound by theory, it is believed that a carboxybinder group allows more efficient electron transfer to particles 119than phosphonic acid binder group 126.

In general, particle surface group 121 can be associated with particles119 as desired.

Typically, the method includes forming a mixture of the particle surfacegroups 121, particles 119, and a solvent (e.g., an organic solvent suchas CH₂Cl₂). Particles 119 may be fixed (e.g., with respect to surface131 of layer 103) or free. In some embodiments, the surfaces of theparticles include a coating that remains from their synthesis. Themixture is allowed to react for a period of time sufficient to displacethe coating (if present) and associate the particle surface groups withthe particles. In general, the time is from about 3 hours to about 24hours (e.g., from about 10 hours to about 16 hours, such as about 12hours). Generally, the temperature is from about 10° C. to about 80° C.(e.g., from about 15° C. to about 50° C., such as about 30° C.). Thereaction can be carried out in an inert environment (e.g. beneath awater-free atmosphere, such as dry nitrogen and/or argon). Afterallowing the mixture to react, a second organic solvent (e.g., methanolor ethyl acetate) is added to the mixture to precipitate particles 119with associated particle surface groups 121. Displaced coating andexcess particle surface groups are removed from the precipitatedparticles by washing with the second solvent.

Particles 119 with particle surface 121 can be combined with photoactivebinder 117 and LC material 200 to form photoactive layer 113.

Photoactive binder material 117 is typically a conductive polymer ableto transport electrons. Examples of photoactive binder materials includepolythiophene, polythiophene derivatives (e.g.,poly-(3-hexylthiophene)), and other polymers discussed above withrespect to the particle surface group and below with respect to the LCmaterial.

Turning now to LC material 200, FIG. 4 shows that the LC material is adiscotic nematic LC material. The term discotic, as used herein, refersto an LC material in which the LC units are substantially flat and/ordisc-like. The term nematic, as used herein, refers to an LC material inwhich the LC units have at least some orientational order but have atleast somewhat random positional order (i.e., their positions are atleast somewhat uncorrelated). We first discuss the orientational orderof LC units 200. As seen in FIG. 4, LC units 201 orient so that theirsymmetry axes a₅ are generally aligned with one another (i.e., axes a₅are generally aligned normal to an inner surface 139 of hole injectionlayer 115). The aligned symmetry axes a₅ define the direction of a layerdirector n of the LC material.

In some embodiments, the maximum average maximum radial dimension d₂ ofLC units 200 is at least about 15 Angstroms (e.g., at least about 20Angstroms, at least about 30 Angstroms, at least about 35 Angstroms). Incertain embodiments, the maximum average diameter d₂ is at most about 40Angstroms (e.g., at most about 35 Angstroms, at most about 30Angstroms). In some embodiments, the maximum average diameter is about18 Angstroms.

Turning to positional order, the positions of LC units that arerelatively closely spaced along the z-axis can exhibit at least somecorrelation. For example, the LC material may include at least somedomains in which LC units align to define columns extending along, forexample, the z-axis (e.g., a column 145). In general, the centers ofmass 229 of the aligned LC units in a column have a maximum offset of nomore than about 70% (e.g., no more than about 50%, no more than about30%, no more than about 20%, no more than about 15%, no more than about10%) of the maximum radial dimension d₂ of the LC units. Hence, theextent of positional ordering can determine the height of the columns.In certain embodiments, the average height of the columns along thez-axis is no more than about 2500 Angstroms (e.g., no more than about1250 Angstroms, no more than about 625 Angstroms, no more than about 300Angstroms, no more than about 200 Angstroms, no more than about 100Angstroms, no more than about 50 Angstroms, no more than about 25Angstroms, no more than about 10 Angstroms).

In some embodiments, the LC material includes domains of LC units thatdefine columns only within a certain maximum distance from surface 131of layer 103 or surface 139 of layer 115. For example, domains of LCunits defining columns may be present within 1000 Angstroms (e.g.,within 500 Angstroms, within 250 Angstroms, within 125 Angstroms) ofsurface 131 and/or surface 139 but not within LC material located atgreater distances from surface 131 and/or surface 139.

In some embodiments, the positions of the LC units are substantiallyrandom (e.g., substantially uncorrelated). For example, in certainembodiments, essentially no columns are formed within LC material (i.e.,the positions of the centers of masses of LC units in adjacent layers ofthe LC material are, on average, substantially random).

Referring to FIG. 5 a, LC units 201 include a plurality of arms 204extending from a central group 202. In general, LC units have at least 2arms (e.g., 3 arms, 4 arms, 5 arms, 6 arms, 7 arms, 8 arms, or more). Atleast some of the arms 204 include a conducting polymer 203, which, inLC unit 201 of FIG. 5 a, is a polythiophene polymer having 4 units(e.g., thiophene rings, vinyl groups, phenyl groups). Each arm includesa plurality of solubilizing groups 204. Examples of solubilizing groupsthat can be used for LC units include solubilizing groups discussedabove with respect to particle surface groups 121.

Conducting polymer 203 of arms 204 and/or central group 202 are able toabsorb light (e.g., solar radiation) that enters photoactive layer 113.As discussed above, the light absorption can produce excitons withinphotoactive layer 113. LC units 201, e.g., central group 202 and arms204 thereof, can typically transfer excitons across at least a portion(e.g., all) of the LC unit. Exciton transfer between LC units isdiscussed below.

At least some (e.g., all) of arms 204 can include one or more lightabsorption modifiers (e.g., one or more sulfoxide groups) to enhanceabsorption of visible solar radiation as discussed above with respect toparticle surface groups 121. For example, at least some (e.g., all) ofthe terminal units of arms 205 may include a light absorption modifier.

While LC unit 201 has been described as having a central group 202formed of a benzene ring and arms 204 each formed of a 4-unit conductingpolymer 203, other LC units can be used. For example, referring to FIG.5 b, an LC unit 275 includes 4 arms 277 and a central group 278. Eacharm 277 includes three units: two 5-member rings and 1 unit defined byfused 5- and 6-member rings. Central group 278 is a benzene ring. Arms277 include a solubilizing group 279. As another example, referring toFIG. 5 c, an LC unit 281 includes 4 arms 283 and a central group 284.Each arm includes three 5-member rings and unit defined by fused 5- and6-member rings. Arms 283 include a solubilizing group 285. Central group284 is a benzene ring. Referring to FIG. 5 d, an LC unit 250 includes acentral group 252 and a plurality of arms 254. Each arm includes aconducting polymer 253 defined by three thiophene units. Arms 254include a plurality of solubilizing groups 124. Central group 252 is afused aromatic ring system. Referring to FIG. 5 e, an LC unit 295includes a central group 296 and a plurality of arms 297. Each arm 297is defined by 5 units. Three units of each arm 297 are thiophene rings,one unit includes fused 5- and 6-member rings, and one unit includes apair of pendent benzene rings.

In general, arms of LC units may have a number of units, such as 3 units(e.g., 4 units, 5 units, 6 units, 7 units, 8 units, 9 units, 10 units,or more). In addition to the arms shown in FIGS. 5 a-5 e, arms of LCunits may be formed of other materials such as any of the units and/orconducting polymers discussed above with respect to conducting polymer122 of particle surface groups 121.

In general, the central group of an LC unit includes one or morearomatic ring systems. Examples of aromatic ring systems include aphenyl group, a napthyl group, a anthracyl group, a pyrenyl group, aterphenyl group, a corenene group, or a combination thereof. A centralgroup can include one or more heteroatoms (e.g., sulfur, oxygen, ornitrogen). For example, a central group can include one or moreheterocyclic ring systems (e.g., 2 heterocyclic rings, 3 heterocyclicrings, 4 heterocyclic rings, 5 heterocyclic rings, 6 heterocyclic rings,or more). In some embodiments, at least some (e.g., all) of theheterocyclic ring systems include sulfur. In some embodiments, thecentral group is free of metal atoms and/or chelated atoms.

In some embodiments, the central group of an LC unit includes smallestring systems of at least two different sizes. For example, in FIG. 5 d,central group 252 of LC unit 250 includes a ring system defined by ananthryl group and 4 fused ring systems each defined by a thiophene ring.Hence, central group 252 has 3 smallest rings of 6 members each (e.g.,the rings of the anthryl group) and 4 smallest rings of 5 members each(e.g., the fused thiophene rings). The fused thiophene rings can broadenthe range of light absorbed by central group 252. As another example, inFIG. 5 e, central group 291 of LC unit 287 includes a benzene ring fusedwith three thiophene rings.

Referring to FIGS. 6 a-6 f, other examples of central groups are shown.In FIG. 6 a, a central group 400 includes a linear chain 401 of fusedbenzene rings and a pair of side groups 402 each having a benzene ring.A substituent group R may be any arm discussed above or another groupsuch as hydrogen, aliphatic, aromatic, or a functional group. Thesubstituents of a central group may be all the same or at least some maybe different from others. In FIG. 6 b, a central group 405 is defined bya triphenylene ring system. In FIG. 6 c, a central group 407 is definedby a polyaromatic ring system having 13 benzene rings. In FIG. 6 d, acentral group 409 is defined by a benzene ring with 4 pendent thiophenerings. In FIG. 6 e, a central group 411 is defined by a benzene ringfused with three thiophene rings. In FIG. 6 f, a central group isdefined by a linear chain of 5 fused benzene rings.

Orientational and/or positional order of the LC units can enhanceelectron and/or hole transfer through photoactive layer 113. Withoutwishing to be bound by theory, it is believed that electron and/or holetransfer between adjacent LC units within a layer (e.g., between LCunits 201 ₁ and 201 ₂ of layer 215 ₂ as seen in FIG. 4) is enhanced bythe interdigitation of arms 204 of the adjacent LC units (FIG. 5). It isalso believed that electron and/or hole transfer between LC units withindifferent layers (e.g., between LC unit 201 ₂ of layer 215 ₂ and LC unit201 ₃ of LC unit 215 ₁) is enhanced by at least some interfacial overlapbetween central groups 202 of the LC units.

Photoactive binder 117, which is present within LC material 200, alsofacilitates transfer of excitons and holes through photoactive layer113. For example, photoactive binder 117 can enhance transfer between LCunits of different layers that lack substantial interfacial overlap oftheir central groups. The energies of the HOMO's and lowest occupiedmolecular orbitals (LUMO's) of arms of the LC units and of thephotoactive binder material 117 are typically similar to furtherfacilitate electron and hole transport.

The orientational order of the LC units can also enhance the performanceof cell 100 by increasing the amount of light absorbed by the LCmaterial. Because the symmetry axes a₅ of the LC units tend to orientalong the z-axis of cell 100, the optical transition moments m₁ of theLC units lie generally in the x-y plane of inner surface 139 of holeinjection layer 115 (the transition moments m₁ are perpendicular to thesymmetry axes as). This orientational order can increase the amount oflight absorption by the LC units by at least 2 times (e.g., at least 3times) as compared to unordered LC units.

As discussed above, LC units 201 can have at least some order (e.g.,orientational order) with respect to inner surface 139 of hole injectionlayer 115. Hence, we discuss the hole injection layer before discussingpreparation of LC material 200.

Hole injection layer 115 is generally formed of a material that, at thethickness used in photovoltaic cell 100, transports holes to electrode111 (FIG. 1). In some embodiments, hole injection layer 115 is a PEDOTpolymer polymerized from an ethyldioxythiophene (EDOT) monomer. Otherexamples of materials from which layer 115 can be formed includepolyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes,polysilanes, polythienylenevinylenes and/or polyisothianaphthanenes. Insome embodiments, hole injection layer 115 can include combinations ofmaterials.

Hole injection layer 115 can be prepared as desired such as by coating(e.g., by spin coating, slot-coating, or roll-to-roll coating). Ingeneral, layer 115 is at least about 25 nanometers thick (e.g., at leastabout 50 nanometers, at least about 75 nanometers, at least about 100nanometers, at least about 150 nanometers thick). In some embodiments,layer 115 is no more than about 200 nanometers thick (e.g., no more thanabout 150 nanometers, no more than about 125 nanometers, no more thanabout 100 nanometers thick).

LC material 200 can be prepared as desired. Typically, a mixtureincluding LC units and, optionally, photoactive binder 117 and/or asolvent (e.g., CHCl₃, THF, chlorobenzene, or combination thereof) isapplied to inner surface 139 of hole injection layer 115. The mixturecan be applied, for example, as a coating. The solvent is allowed toevaporate at a temperature of, for example, from about 10° C. to about80° C. (e.g., from about 15° C. to about 50° C., such as about 30° C.).As the solvent evaporates, intermolecular interactions between differentLC units (e.g., interactions between central groups 202) typicallypromote self-assembly of the LC material on inner surface 139 (FIG. 4).Once a sufficient amount of the solvent evaporates, the orientations andpositions of the LC units are stable with respect to further processing.

The degree of orientational and/or positional order within the LCmaterial can be modified by, for example, changing the nature of the LCunits, the temperature, solvent, and/or rate of solvent evaporation. Forexample, positional order can be increased by using LC units with largerdiameter central groups. The larger central groups increase interfacialattraction between the LC units in adjacent layers (e.g., an LC materialformed of LC unit 250 with central group 252 (FIG. 6) can have morepositional order than LC material 200 formed of LC unit 201 with centralgroup 202 (FIG. 5)). In some embodiments, positional order is increasedby use of large, rigid arms, which can provide positional order even toLC units having a small central group such as a benzene ring. As anotherexample, positional order can be decreased by evaporating the solventmore rapidly because the LC units have less time to move into differentpositions (e.g., more aligned positions) before the solvent evaporates.

Photovoltaic layer 113 can be prepared by contacting the assembled LCmaterial and optional photoactive binder 117 present on the innersurface 139 of hole injection layer 115 with particles 119 ofcounterelectrode 103. The oriented particles extend into the assembledLC material and photoactive binder. Typically, the contacted componentsare thermally laminated so that the LC material, photoactive binder 117,and particles are sandwiched between the hole injection layer 115 andcounterelectrode 103.

Photoactive layer 113 can have a thickness as desired. In general, thethickness of photoactive layer 113 may slightly exceed a maximumdistance of particles 119 from surface 131 of layer 103. In someembodiments, photoactive layer 113 is at least about 50 nanometers thick(e.g., at least about 100 nanometers, at least about 150 nanometers, atleast about 225 nanometers, at least about 300 nanometers thick). Insome embodiments, photoactive layer 113 is no more than about 1000nanometers thick (e.g., no more than about 400 nanometers, no more thanabout 350 nanometers, no more than about 250 nanometers thick).

The lateral dimensions (e.g., the length and width) of cell 100 canprepared as desired. In some embodiments, cell 100 is at least about 2mm wide (e.g., at least about 3 mm, at least about 5 mm, at least about7.5 mm, at least about 10 mm wide). In some embodiments, cell 100 is nomore than about 25 mm wide (e.g., no more than about 15 mm, no more thanabout 10 mm wide). In some embodiments, cell 100 has a strip-like shapeat least about 20 mm long (e.g., at least about 30 mm, at least about 50mm, at least about 100 mm, at least about 200 mm long, at least about1000 mm long). In some embodiments, cell 100 is no more than about 250mm long (e.g., no more than about 20 mm, no more than about 10 mm long).

A plurality of cells 100 can be electrically connected to form a module.For example, a module can include a plurality (e.g., at least 2, atleast 3, at least 4, or more) cells connected in series and/or parallel.

In some embodiments, inner surface 139 of hole injection layer 115 ispatterned to produce surface features that facilitate orientationalordering and/or self-assembly of the LC material. These featuresincrease the tendency of the LC units to orient at the surface. Forexample, in certain embodiments, inner surface 139 is mechanicallymodified before applying the LC units and photoactive binder material tothe surface. The mechanical modification produces minute, orientedfeatures (e.g., scratches) in surface 139. Such mechanical modificationcan be performed, for example, by buffing the inner surface with apolishing cloth such as used for polishing optical surfaces.

In certain embodiments, a chain-like surfactant (e.g., an amphiphile) isapplied to the surface of the hole injection layer. The surfactantchains tend to align normally with respect to the surface. These alignedchains act as surface features that orient the LC units.

In some embodiments, hole injection layer 115 is prepared by apolymerization method that produces a patterned, oriented inner surface.The patterned surface facilitates alignment of LC units 201. In certainembodiments, the polymerization method includes preparing a mixture of amonomer (e.g., EDOT), an electrolyte (e.g., tetraethylammoniumperchlorate (TEAP), and a plurality of amphiphiles (e.g.,poly(oxyethylene)_(n)-oleyl ether (where n is, for example, about 10)).The concentration of monomer is typically about 0.1 M. The concentrationof electrolyte is typically about 0.1 M. Water is added to the mixture,which is then sealed and mixed. The mixture is typically heated to atemperature of from about 25° C. to about 100° C. (e.g., from about 70°C. to about 90° C., such as about 80° C.). The mixture forms a gel uponcooling.

A conductive substrate (e.g., an indium tin oxide (ITO) film on a glasssubstrate) is cleaned (e.g., by sonication and rinsing with an organicsolvent). The gel is applied to the substrate. The amphiphiles withinthe gel tend to orient on the surface forming an LC template. The EDOTmonomer resides within oriented interstices of the LC template. The EDOTis then electropolymerized to prepare a PEDOT layer with a patternedsurface, which has a plurality of substantially perpendicular PEDOTrods. After polymerization, the LC template is washed away (e.g., byrinsing with a solvent such as water) to expose the patterned surface.

The PEDOT layer with patterned surface can be used as the hole injectionlayer of the photovoltaic cell. A mixture including LC units 200,photoactive binder 117, and a solvent is applied to the patternedsurface. The solvent is allowed to evaporate. The oriented rods of thepatterned surface facilitate alignment of the LC units with theirsymmetry axes a₅ generally perpendicular to the PEDOT layer. The exposedLC material and photoactive binder are laminated with particles 119 andcounterelectrode 103 as discussed above.

In some embodiments, a layer 115 with a patterned surface is prepared byphotopolymerization. For example, compounds with one or more doublebonds (e.g., a cinnamate) is deposited on a PEDOT surface and irradiatedwith linearly polarized light. The light is only absorbed by doublebonds aligned with the polarization. Adjacent double bonds polymerize toform a cyclobutane group. The polymerizing molecules bring theirneighbors into alignment. This process of selective absorption andpolymerization forms a patterned surface, which can orient LC unitsapplied thereto.

In some embodiments, a mixture of LC units, photoactive binder 117, anda solvent is applied directly to particles 119 of counterelectrode 103(e.g., the mixture can be applied to the particles before orconcurrently with contacting the mixture and the hole injection layer).The LC units and photoactive binder intercalate among the orientedparticles. The solvent is allowed to evaporate. The hole injection layer115 is laminated over the exposed LC material and photoactive binder toseal the photoactive layer 113.

In some embodiments, the LC material is prepared without use of asolvent. Typically, LC units and, the optional photoactive binder areheated, e.g., to a temperature above the transition temperature of theLC units so that the LC units are able to flow. The heated LC units andoptional binder are applied to surface 131 of layer 103 and/or tosurface 139 of layer 115. In general, the amount of LC units applied tosurface 131 is sufficient to cover particles 119. Layers 103 and 115 aremated to sandwich the LC material therebetween. Upon cooling, the LCmaterial assumes the desired degree of positional and/or orientationalorder. The LC material can be heated before and/or after application tosurface 131 and or surface 139.

Turning now to other components of cell 100, conductive layer 111 isgenerally formed from a layer of a material having an electricalconductivity of at least about 10 (Ω-cm)⁻¹ at 25° C., and can have acomposition and thickness that can be selected based on electricalconductivity, optical properties, and/or mechanical properties asdesired. In some embodiments, the layer 111 can be transparent. Examplesof transparent conductors include certain metal oxides, such as indiumtin oxide (ITO), tin oxide, a fluorine-doped tin oxide, and zinc-oxide.In certain embodiments, layer 111 can be non-transparent (e.g.,substantially opaque). Examples of opaque conductors include acontinuous layer (e.g., a foil) of a metal. In some embodiments, themetal can be, for example, copper, aluminum, titanium, indium, or gold.

In certain embodiments, layer 111 includes a discontinuous layer of aconductive material. For example, layer 111 can include an electricallyconducting mesh. Suitable mesh materials include metals, such aspalladium, titanium, platinum, stainless steel, copper, gold, and alloysincluding such metals. The mesh material can include a metal wire. Theelectrically conductive mesh material can also include an electricallyinsulating material that has been coated with an electrically conductivematerial, such as metal. The electrically insulating material caninclude a fiber, such as a textile fiber or an optical fiber. Examplesof textile fibers include synthetic polymer fibers (e.g., nylons) andnatural fibers (e.g., flax, cotton, wool, and silk). The mesh electrodecan be flexible to facilitate, for example, formation of a photovoltaiccell by a continuous manufacturing process.

A mesh electrode, can take a wide variety of forms with respect to, forexample, wire (or fiber) diameters and mesh densities (i.e., the numberof wire (or fiber) per unit area of the mesh). The mesh can be, forexample, regular or irregular, with any number of opening shapes (e.g.,square, circle, semicircle, triangular, diamond, ellipse, trapezoid,and/or irregular shapes). Mesh form factors (such as, e.g., wirediameter and mesh density) can be chosen, for example, based on theconductivity of the wire (or fibers) of the mesh, the desired opticaltransmissivity, based on the conductivity of the wires (or fibers) ofthe mesh, the desired optical transmissivity, flexibility, and/ormechanical strength. Typically, the mesh electrode includes a wire (orfiber) mesh with an average wire (or fiber) diameter in the range fromabout 1 micron to about 400 microns, and an average open area betweenwires (or fibers) in the range from about 60% to about 95%. A meshelectrode can be formed using a variety of techniques, such as, forexample, ink jet printing, lithography and/or ablation (e.g., laserablation). Mesh electrodes are discussed in U.S. patent application Ser.No. 10/395,823, filed Mar. 23, 2003 and in U.S. patent application Ser.No. 10/723,554, filed Nov. 26, 2003, the disclosures of which are herebyincorporated by reference.

Layer 107 is typically a transparent substrate that insulates conductivelayer 111. Layer 107 can be formed of, for example, glass, polyethylene,or other transparent insulating materials.

Generally, layer 103 is formed of a material having an electricalconductivity of at least about 10 (Ω-cm)⁻¹ at 25° C., and can have acomposition and thickness that can be selected based on electricalconductivity, optical properties, and/or mechanical properties asdesired. Layer 103 can be similar to layer 111. For example, layer 103can be formed from the same materials (e.g., transparent conductor,opaque conductor), can have the same structure (e.g., continuous layer,mesh structure) and can have the same thickness as layer 111. However,it may be desirable for layer 103 to be different from layer 11 in oneor more aspects. For example, in some configurations only one side ofcell 100 (e.g., either the electrode 111 side or the counterelectrode103 side) is exposed to a light source during use, the conductive layerof the unexposed side may be nontransparent material (e.g., opaque).

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.

For example, while generally linear particle surface groups have beendescribed, other configurations are possible. For example, in someembodiments, the particle surface groups are branched. Accordingly,other embodiments are within the scope of the following claims.

1. A photovoltaic cell, comprising: a first electrode; a secondelectrode; and between the first electrode and the second electrode: ahole injection layer; a liquid crystal (LC) material; and a plurality ofparticles in electrical contact with the liquid crystal material.
 2. Thephotovoltaic cell of claim 1, wherein the particles are elongated. 3.The photovoltaic cell of claim 2, wherein the particles are inorganicparticles.
 4. The photovoltaic cell of claim 1, further comprising aplurality of particle surface groups associated with each particle. 5.The photovoltaic cell of claim 4, wherein the particle surface groupsare electroactive.
 6. The photovoltaic cell of claim 4, wherein theparticle surface groups are associated with the particles through acarboxy group.
 7. The photovoltaic cell of claim 4, wherein the particlesurface groups increase an ability of the LC material to wet theparticles.
 8. The photovoltaic cell of claim 4, wherein at least some ofthe particle surface groups comprise a plurality of aromatic rings. 9.The photovoltaic cell of claim 8, wherein at least some of the particlesurface groups comprise a plurality of 5-membered rings, at least someof the rings comprising at least one sulfur atom.
 10. The photovoltaiccell of claim 9, wherein at least some of the particle surface groupscomprise at least one sulfoxide group.
 11. The photovoltaic cell ofclaim 10, wherein one of the 5-membered rings of at least some of theparticle surface groups is a terminal 5-membered ring and only theterminal 5-membered ring comprises a sulfoxide group.
 12. Thephotovoltaic cell of claim 8, wherein at least one of the aromatic ringsof each particle surface group comprises an aliphatic chain comprisingat least 3 carbon atoms.
 13. The photovoltaic cell of claim 1, whereinthe LC material comprises an electroactive LC material.
 14. Thephotovoltaic cell of claim 13, wherein the electroactive LC materialcomprises a nematic LC material.
 15. The photovoltaic cell of claim 14,wherein the nematic LC material is a discotic nematic LC material. 16.The photovoltaic cell of claim 15, wherein the LC material includes aplurality of LC units and at least some of the LC units are in differentlayers of the LC material, the positions of the LC units in differentlayers are substantially random.
 17. The photovoltaic cell of claim 13,wherein the liquid crystal material defines a director and the particlesdefine a major axis longer than a minor axis of the particles, whereinthe director and the major axes of substantially all the particles aresubstantially aligned.
 18. The photovoltaic cell of claim 13, whereinthe liquid crystal material defines a director and the hole injectionlayer comprises a plurality of polymer rods in contact with the liquidcrystal material, wherein the director and substantially all of thepolymer rods are substantially aligned.
 19. The photovoltaic cell ofclaim 13, wherein the liquid crystal material comprises a plurality ofdiscotic LC units and each LC unit comprises an aromatic central groupand at least 4 electroactive arms.
 20. The photovoltaic cell of claim19, wherein the aromatic central group comprises at least 2 fusedaromatic 6-membered rings and at least 4 heterocyclic rings fused to thearomatic 6-membered rings.
 21. The photovoltaic cell of claim 20,wherein the aromatic central group comprises at least 3 fused aromatic6-membered rings.
 22. The photovoltaic cell of claim 20, wherein atleast some of the heterocyclic rings are 5-membered rings comprising asulfur atom.
 23. The photovoltaic cell of claim 19, wherein the discoticLC units are free of metal atoms.
 24. The photovoltaic cell of claim 19,wherein at least some of the electroactive arms comprise a plurality of5-membered rings, at least some of the 5-membered rings comprising asulfur atom.
 25. The photovoltaic cell of claim 24, wherein at least oneof the 5-membered rings of each electroactive arm comprises an aliphaticchain comprising at least 3 carbon atoms.
 26. The photovoltaic cell ofclaim 1, further comprising a photoactive binder dispersed among theliquid crystal material.
 27. The photovoltaic cell of claim 26, whereinthe photoactive binder comprises a polymer that has a LUMO similar to aLUMO of the LC material.
 28. The photovoltaic cell of claim 26, whereinthe photoactive binder comprises a polymer that has a HOMO similar to aHOMO of the LC material.
 29. The photovoltaic cell of claim 1, whereinthe first electrode is a mesh electrode.
 30. The photovoltaic cell ofclaim 1, wherein the particles comprise a material selected from thegroup consisting of ZnO, WO₃, and TiO₂.
 31. The photovoltaic cell ofclaim 30, wherein the particles are elongated and a first end of atleast some of the particles is fixed with respect to one of theelectrodes and a second end of the at least some of the particles isfree of the electrode.
 32. The photoactive cell of claim 31, wherein atleast some of the particles comprise at least one particle surface groupassociated therewith, a photoactive material is dispersed within the LCmaterial, and the second end of the at least some particles extends intothe LC material and photoactive binder.
 33. A method for manufacturing aphotovoltaic cell, comprising: disposing a hole injection material, aplurality of particles, at least some of which comprise a particlesurface group associated therewith, and a plurality of liquid crystal(LC) units between first and second electrodes.
 34. The method of claim33, wherein the particles are elongated particles.
 35. The method ofclaim 34, wherein the elongated particles are substantially orientedrelative to one another.
 36. The method of claim 33, comprising forminga discotic nematic LC material comprising the LC units.
 37. The methodof claim 36, wherein forming the discotic nematic LC material comprises:forming a layer of the hole injection material, the layer having asurface; associating a surfactant with a surface of the layer of thehole injection material; and contacting the surfactant with the LCunits.
 38. The method of claim 36, wherein forming the discotic nematicLC material comprises: forming a plurality of oriented projectionscomprising the hole injection material; and contacting the LC units withthe plurality of oriented projections.
 39. The method of claim 38,wherein forming the plurality of oriented projections comprises: forminga plurality of interstices within an surfactant adjacent a surface; andpolymerizing a polymerizable precursor to the hole injection materialwithin the interstices to form the plurality of oriented projections.40. The method of claim 39, wherein the polymerizable precursorcomprises a monomer of polythiophene or derivative thereof.
 41. Themethod of claim 36, wherein the particles are elongated particles thatare substantially aligned with one another and forming the discoticnematic LC material comprises contacting the LC units with the pluralityof elongated particles.
 42. The method of claim 33, comprisingassociating an electroactive surfactant material with at least some ofthe particles.
 43. A discotic nematic liquid crystal (LC) material,comprising: a plurality of LC units, each LC unit comprising: a centralmoiety comprising at least one aromatic ring; a plurality ofelectroactive arms extending outward from the central moiety, each armcomprising a plurality of cyclic groups at least some of which compriseat least one sulfur atom.
 44. The discotic nematic LC material of claim43, wherein the central moiety comprises a plurality of fused aromaticrings.
 45. The discotic nematic LC material of claim 43, wherein atleast some of the cyclic groups comprise a sulphoxide group.
 46. Thediscotic nematic LC material of claim 43, wherein the central moiety isfree of metal.
 47. The discotic nematic LC material of claim 43, furthercomprising an amount of a photoactive binder distributed among the LCunits and in electrical communication therewith.
 48. The discoticnematic LC material of claim 47, wherein the photoactive bindercomprises a polythiophene compound.
 49. The discotic nematic LC materialof claim 43, wherein the central moiety comprises at least 3 fusedrings, each of the fused rings comprising 6 carbon atoms.
 50. Thediscotic nematic LC material of claim 49, wherein the central moietycomprises at least 4 heterocyclic rings, each heterocyclic ring beingfused to one of the 3 fused rings and comprising a sulfur atom.
 51. Aphotovoltaic cell comprising the discotic nematic LC material of claim43.
 52. The photovoltaic cell of claim 51, further comprising aplurality of nanorods each having a major axis, the discotic nematic LCmaterial defining a director, the long axis of the nanorods and thedirector being substantially aligned.